Hydraulic fracturing is a process of initiating and subsequently propagating a fracture in a geologic formation through utilization of fracturing fluid. To create the fracture in the geologic formation, a drill is employed to create a well bore that reaches depths of several thousand feet (until a desired geologic formation is reached). A well casing is placed in the well bore. The well casing is typically composed of steel and is cemented in place to stabilize the well casing with respect to the Earth.
Hydraulic fracturing is commonly employed to enhance the fluid flow permeability of shale geologic formations for petroleum (oil and/or natural gas) and geothermal energy production. Subsequent to the well casing being cemented in place, a fracturing fluid is pumped down the well bore and through perforations in the well casing at a pressure that is in excess of the fracture gradient of the geologic formation. Such pressure causes the geologic formation to fracture. Pumping of the fracturing fluid down the well bore is continued to extend the fracture further into the formation. As the fracture extends, a proppant is added to the fracture fluid and pumped down the well bore and into the fracture, thereby propping the fracture open when pumping of the fracture fluid ceases. This causes the geologic formation to become permeable via the fracture, thereby allowing natural gas or oil to be extracted from the geologic formation. Hydraulic fractures can be induced using vertical, horizontal and/or slanted wells. This process is commonly referred to as hydraulic fracturing.
Because a typical fracture and proppant placement occur thousands of feet beneath the surface of the Earth and because a propped fracture can extend from the well bore in a variety of directions and orientations, it is difficult to determine the location of a fracture and proppant within the geologic formation. Modeling techniques have been developed that attempt to use electromagnetic fields generated in the fracture and measured at the surface of the Earth to locate the proppant and fractures. Although these modeling techniques have been successful in helping to locate induced fractures and proppant, they can be limited by the dispersion and attenuation of the electromagnetic waves (e.g., electromagnetic waves having a frequency of less than 100 kHz) as they pass from the fracture, through various geologic media, to the surface of the Earth.
In particular, absorption of some of the generated fields can attenuate the signal that is eventually received at the surface. Because waves at different frequencies in a wave packet travel with different velocities, dispersion can cause an input signal to spread so that relatively high frequency input signal components are indistinguishable when detected at the surface. If care is not taken, dispersion and attenuation of electromagnetic signals from a fracture can negatively affect the accuracy with which the location of the fracture and proppant can be determined.
It would therefore be desirable to provide improved systems and methods for evaluating well hydraulic fracturing and completion techniques useful in extracting natural gas, oil and geothermal reserves from a geologic formation and specifically determine within reasonable error the spatial extent of proppant placed in the fracture under a specific set of hydraulic fracturing operating parameters.